1. Field of the Invention
The invention relates to a thin film transistor and a method of producing a thin film transistor.
2. Description of the Related Art
In recent years, amorphous In—Ga—Zn—O-based homologous oxide materials (hereinafter, referred to as “IGZO-based oxide materials” or simply as “IGZO” sometimes), represented by a composition formula In2-xGaxO3(ZnO)m (0<x<2 and m is a natural number) in a crystalline state thereof, have been attracting attention.
Hosono et al. from the Tokyo Institute of Technology reports that amorphous IGZO-based oxide materials exhibit a value of electrical resistivity similar to that of a semiconductor and can be formed into a film at room temperature, and that these materials can achieve a mobility that is equal to or higher than that of amorphous silicon (Hosono et al., Non-patent Document 1, Nature, 432 (2004), pp. 488-492).
In particular, amorphous IGZO-based oxide materials represented by the above composition formula in which m=1 are highly promising material systems because of the large ratio of overlapping of electron orbits between In—In, which is considered to contribute to the conduction of electrons.
Research and development on amorphous IGZO-based oxide materials is being intensely conducted with respect to the use of these materials as, for example, an effective material for an active layer of a thin film transistor (hereinafter, also referred to as a “TFT”).
On the other hand, IGZO-based oxide materials having a crystalline structure are described, for example, in the following literature.
Non-patent Document 2 (Journal of the American Ceramic Society, 82 (1999), pp. 2705-2710) describes a method of producing a crystalline IGZO-based oxide material in which m=1, in which a mixture of raw materials including In, Ga and Zn is annealed at 1350° C. or higher, and then rapidly cooled down from this extremely high annealing temperature. The document also discloses that the solid solution range of Ga (range of x) is from 0.66 to 1.06.
Further, Japanese Patent No. 3947575 discloses a process of subjecting a crystalline IGZO-based oxide material in which m=1, which is obtained by annealing at certain conditions, to a reduction heat treatment at a certain temperature in a hydrogen or argon atmosphere.
In this regard, in many electron-conductive oxide materials including IGZO, properties that are unique to these materials are significantly affected by the value of the oxygen deficit amount δ. When the value of δ is great, a large number of carriers (electrons) are generated and a “degenerate semiconductor”, having a Fermi level within the conductive band, is formed. In other words, the oxide material in this state is a conductor that exhibits metallic conductivity. On the other hand, when the value of δ is small, generation of carriers can be suppressed and the oxide material can exist as a semiconductor. The above fact indicates that the nature of an oxide material may greatly vary between a conductor and a semiconductor, depending on the value of the oxide deficit amount δ.
In the method described in Non-patent Document 2, the purpose of rapid cooling from a high-temperature range (quenching) is to obtain an oxide material that also maintains its state achieved at high temperature at room temperature. Generally, oxygen bound to an oxide material escapes more easily at a higher temperature, from the viewpoint of chemical equilibrium, whereby the value of δ is increased. Accordingly, an IGZO-based oxide material having a greater value of δ may be obtained as a result of performing quenching, and an IGZO-based oxide material having a greater value of δ cannot suppress the generation of carriers and behaves as a degenerate semiconductor, i.e., a metal (conductor).
On the other hand, in the method described in Japanese Patent No. 3947575, the reduction heat treatment is performed in order to introduce oxygen deficits into an oxide material. Therefore, the IGZO-based oxide material obtained by this method has a great value of δ, thereby increasing the carrier concentration thereof. As a result, the IGZO-based oxide material obtained by this method behaves as a degenerate semiconductor, i.e., a metal (conductor).
As described above, both Non-patent Document 2 and Japanese Patent No. 3947575 indicate that a crystalline IGZO-based oxide material in which m=1 is a conductor, rather than a semiconductor. Therefore, if an IGZO-based oxide material in which m=1 can be obtained as a semiconductor, this material can be widely used in electronic devices, such as as an active layer of a TFT.
Further, if the amount of Ga in an IGZO-based oxide material in which m=1 is not appropriate, this material does not form a single phase of IGZO but includes multiple crystal phases therein. Therefore, electron scattering tends to occur at grain boundaries or the like from a microscopic viewpoint. Accordingly, in order to apply a crystalline IGZO-based oxide material in which m=1 to electronic devices, an IGZO-based oxide material in which the amount of Ga is within the solid-solution range and a single phase of IGZO is formed, as described in Non-patent Document 2, is desired in view of maintaining the mobility of carriers (electrons).